Packaged circuit boards for electronic instruments are typically coated by a moisture-proof insulator film to protect the circuit boards from moisture, electric leakage and dust. Preferably, the moisture-proof insulator films are what are known as conformal coatings, such as acrylic, polyurethane or epoxy synthetic resins dissolved in a volatile solvent. When applied to a clean printed circuit board, a uniform thickness insulative resin film is formed as the solvent evaporates on a continuous basis.
In the past, five principal methods have been used to apply coatings of moisture-proof insulators to printed circuit boards. These methods are discussed in prior U.S. Pat. No. 4,753,819, entitled "Method For Applying A Moisture-Proof Insulator Coating on Packaged Circuit Boards", in the name of Takagi Shimada; and, in the continuation-in-part of such patent, namely, U.S. Pat. No. 4,880,663 in the name of Takagi Shimada, entitled "Method of Applying A Moisture-Proof Insulative Coating To Printed Circuit Boards Using Triangular or Dovetail-Shaped Liquid Film Emitted From A Flat-Pattern Nozzle". The disclosures of these patents are hereby incorporated by reference in their entirety herein.
As discussed in such patents, the principal methods of applying coatings of moisture-proof insulators to printed circuit boards include:
(a) the immersion method, in which packaged circuit boards are immersed in an immersion tank containing the moisture-proof insulator; PA1 (b) the brush coating method, in which the moisture-proof insulator is applied manually by a brush to the printed circuit board; PA1 (c) the roller method, in which a sheep's wool roll impregnated with a moisture-proof insulator is rolled onto the surface of the printed circuit board to coat it; PA1 (d) the spray method, in which the moisture-proof insulator is applied to the printed circuit board by spraying techniques; and PA1 (e) the slit die method, in which the moisture-proof insulator is pressurized and extruded from the slit die to eject a film for coating the printed circuit board surface.
As discussed in the above-referenced Shimada patents, each of the foregoing methods have certain advantages and disadvantages. For example, all methods except brush coating require masking for those parts of the printed circuit board to be left uncoated. The mounting and removal of masks from the board must be done manually which can create a bottleneck in the mass production of circuit boards. Brush coating, while not requiring masking, is labor-intensive and otherwise unsuitable for mass production.
In order to satisfy demand, the most commonly used insulative coating method employed in mass production is the spraying method. Each of the above-referenced Shimada patents discloses a method of spraying insulative liquid coating material onto a printed circuit board in which a flat pattern nozzle is employed and relative movement is effected between the nozzle and circuit board. Commonly, an external heater is employed to heat the coating material prior to its introduction into the dispenser. In mass production, robot arms have often been employed to manipulate the spray guns having spray nozzles which are mounted in a fixed position thereon. These robot arms are capable of moving the spray gun and its spray nozzle in a Z direction, i.e., toward and away from the printed circuit board, and in the X and Y directions, i.e., along the length and width of the circuit board.
The method and apparatus disclosed in the Shimada U.S. Pat. No. 4,753,819 and U.S. Pat. No. 4,880,663 results in the production of a "leaf-shaped" pattern of coating material which is emitted from the discharge outlet of the nozzle of the spray device. This leaf-shaped pattern has a width dimension which increases in a direction from the discharge outlet of the nozzle outwardly to a maximum width, and then tapers inwardly back to a minimum width. Accordingly, the width of the pattern of coating material which is applied to the circuit board is dependent upon the spacing between the discharge outlet of the nozzle and the circuit board. This spacing can be varied as required for a given circuit board configuration with robot arms capable of moving the spray device and nozzle in the Z direction, as noted above, i.e., toward and away from the circuit board.
The problem with varying the width of the spray pattern on the circuit board by adjusting or varying the distance between the nozzle and circuit board along the Z axis is that an uneven thickness of coating material is applied to the circuit board. This is because the same flow or quantity of coating material is discharged from the nozzle regardless of its position relative to the circuit board. A thinner layer of coating material is applied to the circuit board when the width dimension of the spray pattern is greatest because the coating material is applied over a larger area. A proportionately thicker layer of coating material is applied by the spray nozzle when the width of the pattern is narrow because the same quantity or flow of coating material is discharged onto a smaller area. In many instances, it is preferable to obtain a coating of uniform thickness across the entire circuit board to ensure the components thereon are all properly coated and that coating material is not wasted.
Another problem area in the application of coating material onto circuit boards involves changing the direction of movement of the spray gun in the X and Y directions. In the past, in order to ensure that the desired pattern of coating material is applied by the spray nozzle in both the X and Y directions of movement of the spray gun, it has been necessary to reorient the spray nozzle, and thus the spray gun, 90.degree. each time the direction of movement is changed from the X direction to the Y direction or vice versa. Rotation of the entire spray gun requires relatively heavy mechanisms which often are difficult to support on a robot arm and/or which reduce the speed of movement of the robot arm. Not only have such mechanisms proven to be cumbersome, but they are capable of reorienting the spray gun at only 90.degree. increments so that the spray pattern is properly aligned for movement along either the X axis or Y axis. Additionally, in some applications, it is desirable to apply the coating material along a non-linear path, e.g., circular or semicircular, instead of a straight line path along an X or Y axis. Because robot manipulators of the type described above are only capable of orienting the spray device at 90.degree. increments, non-linear patterns cannot be obtained without unduly overlapping the coating material thus producing a layer on the printed circuit board having a non-uniform thickness.
Another difficulty associated with prior art circuit board coating devices is that the circuit boards are not necessarily planar because circuit components and the like can protrude from the surface thereof. In some applications, it is desirable to coat the vertical sides or underneath portion of a component carried on a circuit board. This cannot readily be accomplished with robot arms capable of moving a spray gun solely in the X, Y and Z directions.